A wheel-support hub unit is used for supporting a wheel of an automobile so as to be able to rotate freely with respect to a suspension. The construction of a rolling bearing unit in which a rotating bearing ring is supported on the inner-diameter side or outer-diameter side of a stationary bearing ring is employed as the basic construction of a wheel-support hub unit. FIG. 11 illustrates the construction of a wheel-support hub unit that is disclosed in JP 2003-74570 (A). This wheel-support hub unit 1 has: an outer ring 2, which is the stationary bearing ring; a hub 3, which is the rotating bearing ring; and a plurality of rolling elements 4 for supporting the hub 3 so as to be able to freely rotate around the outer ring 2.
The outer ring 2 has an outer-ring main body 5 and pair of outer-ring raceway members 6. The outer-ring main body 5 is a cylindrical shape having a stationary-side flange 7 that protrudes outward in the radial direction and is formed around a portion of the outer circumferential surface of the outer-ring main body 5 near the inside end in the axial direction. Each of the outer-ring raceway members 6 has an outer-ring raceway 8 that is formed around the inner circumferential surface thereof, and is fastened on the inside of the outer-ring main body 5. The outer ring 2, having this kind of construction, is supported by and fastened to the suspension (not illustrated in the figure) by connecting the stationary-side flange 7 to a support member such as a knuckle, axle housing, axle beam or the like of the suspension so as not to be able to rotate with respect to the suspension during use. Here, the “inside” in the axial direction refers to the inside in the width direction of the automobile body when assembled in an automobile, and corresponds to the right side in each of the drawings. On the other hand, the “outside” in the axial direction refers to the outside in the width direction of the automobile, and is the left side in each of the drawings.
The hub 3 has a hub main body 9, and outside inner-ring raceway member 10, and an inside inner-ring raceway member 11. A rotating side flange 12 for supporting a wheel is formed around the outer circumferential surface of the hub main body 9 in a portion near the outside end that protrudes outward in the axial direction further than the outside end section in the axial direction of the outer ring 2. Moreover, a screw section is formed around the outer circumferential surface of the hub main body 9 on the inside in the axial direction. On the other hand, the outside inner-ring raceway member 10 has an outside inner-ring raceway 13 that is formed around part of the outer circumferential surface thereof, and is fixed onto the middle section in the axial direction of the hub main body 9. The inside inner-ring raceway member 11 has an inside inner-ring raceway 14 that is formed around the outer circumferential surface thereof, and a shoulder section 15 that is formed in a portion that is adjacent to the inside in the axial direction of the inside inner-ring raceway 14. The inside inner-ring raceway member 11, with the inner circumferential surface thereof fixed onto a portion of the outer circumferential surface of the hub main body 9 near the inside end in the axial direction, is fastened to the hub main body 9 by a nut 16 that is screwed around the screw section of the hub main body 9. The hub 3, having this kind of construction, rotates together with a wheel (not illustrated in the figure) that is supported by and fastened to the rotating-side flange 12 when in use.
The rolling elements 4 are arranged such that there is a plurality in each row between the outside inner-ring raceway 13 and the inside inner-ring raceway 14 and the outer-ring raceway 8, and are held in place by a retainer 17 so as to be able to roll freely.
The opening on the outside end in the axial direction of the rolling-element installation space 18 that exists between the inner circumferential surface of the outer ring 2 and the outer circumferential surface of the hub 3 is covered by a seal ring 19 that is mounted in this opening section. On the other hand, the opening on the inside end in the axial direction of the outer ring 2 is covered by a cover 21 that is mounted in this opening section. The seal ring 19 and the cover 21 prevent foreign matter such as dust or rainwater from entering into the rolling-element installation space 18, and prevent grease that is filled in the rolling-element installation space 18 from leakage to the outside.
Incidentally, in order to improve the operating performance of the automobile such as riding comfort and operating stability of the automobile, the unsprung weight of the automobile is reduced by making the suspension to which the outer ring is fastened and the wheel that is fastened to the hub using an aluminum alloy instead of a ferrous alloy such as carbon steel.
Furthermore, it is proposed that, together with reducing the weight of the wheel-support hub unit itself by using an aluminum alloy for the outer ring 2 and hub 3 material as well, the occurrence of galvanic corrosion in the portions where the suspension comes in contact with the wheel and wheel-support hub unit be prevented by combining this wheel-support hub unit with the aluminum alloy suspension and wheel. It is also possible to use a synthetic resin that contains reinforced fiber as a material of the wheel-support hub unit.
On the other hand, from the aspect of maintaining the durability of the wheel-support hub unit, there is a need to maintain high strength of the outer-ring raceways that are formed around the outer ring 2, and the inner-ring raceways that are formed around the hub 3. In order to accomplish this, together with manufacturing the outer-ring main body 5 and hub main body 9 using an aluminum alloy or synthetic resin that contains reinforced fibers, the outer-race raceway members 6, outside inner-race raceway member 10 and inside inner-raceway member 11 around which the raceway surfaces are formed are manufactured using a ferrous alloy such as carbon steel or bearing steel. As a result, together with being able to reduce the weight of the wheel-support hub unit, it is possible to maintain the strength of the inner-ring raceways and outer-ring raceways, and to improve the durability of the overall wheel-support hub unit.
However, the temperature of the outer ring 2 and the hub 3 increases during use. Therefore, due to differences in the linear expansion coefficients between the outer-ring raceway member 6 and outer-ring main body 5, or between the outside inner-ring raceway member 10 and inside inner-ring raceway member 11 and the hub main body 9, which are made of different materials, there is a possibility that preloading that is applied to the rolling elements 4 may change, or gaps may occur in the areas of fit between these members.
In the case where such gap occurs, in the construction of FIG. 11, the boundary 24 between the hub main body 9 and the outside inner-ring raceway member 10 is located further on the outer space side than the seal ring 19, so the boundary 24 where the gap occurs becomes a pathway for rainwater or dirty water to enter. When water enters in between the hub main body 9 and the outside inner-ring raceway member 10 that are made of different metals, galvanic corrosion occurs in the hub main body 9 made of aluminum alloy, the electric potential of which is lower than ferrous alloy. The contents of JP 2003-74570 (A) are incorporated into this specification by reference.